These are some important links for 4 and 8GB of memory. I have tried to consolidate them so there are not so many stickies in this forum.

8000 Platinum, 8000 Reaper X, or 8500 Reaper tRFC 54 SPD flashing instructions

The importance of tRFC and 4/8GB of memory

4/8GB PC6400 DDR2 Platinum Kit issues...Look here

Vista and 4GB on system Memory Info here

4gb of Memory with 32-bit Windows XP or Vista.